1. Field of the Invention
This invention relates to iron-rich metallic glass alloys having the combination of high saturation induction and high Curie temperatures, which results in superior soft ferromagnetic properties.
2. Description of the Prior Art
Glassy metal alloys (metallic glasses) are metastable materials lacking any long range order. They are conveniently prepared by rapid quenching from the melt using processing techniques that are conventional in the art. Examples of such metallic glasses and methods for their manufacture are disclosed in U.S. Pat. Nos. 3,856,513, 4,067,732 and 4,142,571. The advantageous soft magnetic characteristics of metallic glasses, as disclosed in these patents, have been exploited in their wide use as materials in a variety of magnetic cores, such as in distribution transformers, switch-mode power supplies, tape recording heads and the like.
Applications for soft magnetic cores, in a particular class now receiving increased attention, are generically referred to as pulse power applications. In these applications, a low average power input, with a long acquisition time, is converted to an output that has high peak power delivered in a short transfer time. In the production of such high power pulses of electrical energy, very fast magnetization reversals, ranging up to 100 T/.mu.s, occur in the core materials. Examples of pulse power applications include saturable reactors for magnetic pulse compression and for protection of circuit elements during turn on, and pulse transformers in linear induction particle accelerators.
Metallic glasses are very well suited for pulse power applications because of their high resistivities and thin ribbon geometry, which allow low losses under fast magnetization reversals. (See, for example, (i) "Metallic Glasses in High-Energy Pulsed-Power Systems", by C. H. Smith, in Glass . . . Current Issues, A. F. Wright and J. Dupuy, eds., (NATO ASI Series E, No. 92, Martinus Nijhoff Pub., Dordrecht, The Netherlands, 1985) pp. 188-199.) Furthermore, metallic glasses, due to their non-crystalline nature, bear no magneto-crystalline anisotropy and, consequently, may be annealed to deliver very large flux swings, with values approaching the theoretical maximum value of twice the saturation induction of the material, under rapid magnetization rates. These advantageous aspects of metallic glass materials have led to their use as core materials in various pulse power applications: in high power pulse sources for linear induction particle accelerators, as induction modules for coupling energy from the pulse source to the beam of these accelerators, as magnetic switches in power generators for inertial confinement fusion research, and in magnetic modulators for driving excimer lasers.
Reference has been made to annealed samples in the discussion above. It is a well known fact in the art that metallic glasses have to be subjected to anneals (or, synonymously, heat treatments), usually in the presence of external magnetic fields imposed on the materials, before they display their excellent soft magnetic characteristics. The reason for these required anneals is that as-cast ribbons of metallic glasses tend to have high quenching stresses, resulting from the very rapid cooling rates employed to cast these materials. In the case of ferromagnetic metallic glasses, these stresses lead to a distribution of stress-induced magnetic anisotropy, which, in turn, tends to mask the true soft ferromagnetic properties realizable from these materials. To remedy this situation, metallic glasses must be annealed at suitably chosen temperatures, for appropriate time intervals, whereby the quenching stresses are relaxed while the glassy structure of these materials is preserved.
The purpose of the externally imposed fields during anneals is to induce a magnetic anisotropy, i.e., a preferred direction of magnetization. Accordingly, the anneal temperatures are chosen to be very close to the Curie temperatures of the materials, so that small, and practical, strengths (up to about 1600 A/m) may be used for the external fields. Since the beneficial effects due to annealing, such as stress relaxation, are a result of kinetic processes, a higher Curie temperature in the material allows for high anneal temperatures and therefore, shorter anneal times. Furthermore, a low anneal temperature with a longer anneal time may yet not fully relax the stresses, and a preferred anisotropy direction may not be fully established.
Another advantage of a higher Curie temperature in a ferromagnetic material is that the rate of reduction of the saturation induction with temperature is reduced, so that higher induction levels are available in the material at given device operating temperatures or, for a given induction level, the material may be driven to higher operating temperatures.
Most pulse power applications require a high saturation induction in the core material, which leads to large flux swings in the core. The core material should, preferably, also possess a low induced magnetic anisotropy energy. A low magnetic anisotropy energy leads to lower core losses, by facilitating the establishment of an optimal ferromagnetic domain structure, and therefore allows the cores to operate with greater efficiency.
High saturation induction levels are necessary in other applications for metallic glasses as well. Requirements for miniaturization of electronic components in, say, switch-mode power supplies, will be met by higher saturation induction levels, and line frequency distribution transformers may be designed to operate at higher induction levels.
METGLAS.RTM. 2605CO (nominal composition: Fe.sub.66 Co.sub.18 B.sub.15 Si.sub.1), available from Allied-Signal Inc., is a high induction metallic glass alloy currently used in many of the pulse power applications recited above. This metallic glass is disclosed in U.S. Pat. No. 4,321,090, wherein metallic glasses having a high saturation induction are disclosed. The saturation induction of this glassy alloy, in the annealed state, is about 1.8 T. However, the high cobalt content in this alloy imparts a high value for the magnetic anisotropy energy and, consequently, high core losses. The value of about 900 J/m.sup.3 for the magnetic anisotropy energy in this alloy is among the highest obtained in metallic glasses. In spite of its high induction, a maximum flux swing of only about 3.2 T is attainable from this alloy. Furthermore, the high Co content in this alloy leads to high raw material costs. Considering that cores used in pulse power applications may contain as much as 1000 kg of core material per core, and considering that Co had been classified as a strategic material, a more economical alloy containing substantially reduced levels of Co is highly desirable.
A metallic glass alloy that contains no cobalt is METGLAS.RTM. 2605SC (nominal composition: Fe.sub.81 B.sub.13.5 Si.sub.3.5 C.sub.2), available from Allied-Signal Inc. This alloy is disclosed in U.S. Pat. No. 4,219,355. The low magnetic anisotropy energy (about 100 J/m.sup.3) of this alloy has been exploited in a variety of applications, including certain pulse power applications. However, this alloy has a lower saturation induction (about 1.6 T in the annealed state) and a relatively low Curie temperature of about 620 K., when compared to other Fe-B-Si metallic glasses in the prior art.
A metallic glass alloy that offers a combination of high saturation induction, high Curie temperature and low anisotropy energy would be highly desirable for the purposes of many applications. An additional advantage would be derived if such an alloy were to offer economy in production costs.